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From: RayeR <glaux AT centrum DOT cz>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: running ring0 code under DJGPP
Date: Sun, 02 Sep 2007 10:19:58 -0700
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BTW I found that
__dpmi_set_descriptor(ldt_cgsel,&ldt_callgate_descriptor);
DJGPP function works wrong way! I allocated LDT descriptor with
ldt_cgsel=__dpmi_allocate_ldt_descriptors(1)
and prepared descriptor structure with all needed attributes like
pl=3, system=0, sel:offset etc.
call __dpmi_set_descriptor() to transfer local structure to LDT but
when I read LDT entry back
it was not exact as I set. System bit was set 1 and could be 0.
So then I used movedata(_my_ds(),
(DWord)p_ldt_callgate_descriptor,ldt_ldtsel,ldt_callgate_selector.index*8,8);
to copy structure into LDT entry;

So now I can create and set various segments/callgates in LDT
my idea is:
1) crate CS alias segment but with DPL=0, execute-read to hold my
ring0 code (part of my whole program)
2) create CS alias selector pointing CS alias descriptor. Then
CS_alias:offset would be regular pointer to ring0 code
3) create callgate in LDT, PL=3, execute-only-conforming pointing the
CS_alias:offset where offset is pointer to inline
assembly ring0 code rdmsr ending with retf
4) create callgate selector pointing LDT callgate descriptor with PL=3
5) make far PM pointer consist of callgate selector:zero offset
6) call this PM pointer

result: crashed with GPF :(